The TI 73 series is a series of graphing calculators made by Texas Instruments, all of which have identical hardware.
The original TI-73 graphing calculator was originally designed in 1998 as a replacement for the TI-80 for use at a middle school level (grades 6-9). Its primary advantage over the TI-80 is its 512 KB of flash memory, which holds the calculator's operating system and thereby allows the calculator to be upgraded. Other advantages over the TI-80 are the TI-73's standard sized screen (as opposed to the TI-80's smaller screen), the addition of a link port, 25 KB of RAM (as compared to the TI-80's 7 KB of RAM), and a faster 6 MHz Zilog Z80 processor (as compared with the TI-80's 980 kHz proprietary processor). The TI-73 also uses the standard 4 AAA batteries with a lithium backup battery (instead of the TI-80's 2 CR2032 lithium batteries).
In 2003, the TI-73 was redesigned with a new body shape and redesignated the TI-73 Explorer to indicate its currently intended use as a bridge between the TI-15 Explorer and similar calculators and the TI-83 Plus, TI-84 Plus, and similar calculators. Later, the TI-73 Explorer was remodeled to resemble the TI-84 Plus graphing calculator more closely.
Due to lack of demand in middle schools, the TI-73 and TI-73 Explorer have not been huge sellers for TI and are not carried by most retail stores. Most American upper-level middle school algebra courses generally tend to use the TI-83 or TI-84 families instead of the TI-73 or TI-73 Explorer, while most basic middle school math courses generally do not use graphing calculators, instead opting for scientific calculators such as the TI-30 or TI-34 families.
Originally the TI-73 could only run programs written in TI-BASIC, although that has changed in recent years. In 2005, an assembly shell called Mallard was released for the TI-73. Mallard allows the user to run programs written in assembly language. As with the TI-82 and the TI-85 before, a hacked backup file is downloaded containing the assembly shell.